The invention relates to an electronic trip device connected to means for measuring the current flowing in a polyphase electrical power system to be protected and comprising means for sampling current signals supplied by the means for measuring, means for calculating connected to the means for sampling and calculating a quantity representative of the thermal state of the power system, and means for comparing said quantity to preset tripping thresholds, the means for calculating comprising first means for filtering, of the finite impulse response filter type, receiving the current signal samples and supplying signals representative of the rms value of the current.
In certain known microprocessor-based electronic trip devices, certain tripping functions, notably long delay tripping, are achieved by means of a digital finite impulse response (FIR) filter. Such a filter enables a predetermined number of samples to be summed. If the input samples of the filter are constituted by the squares of the samples representative of the measured current, signals representative of the rms value of the measured current are obtained on output. This type of filter does not however always enable the required frequency response to be obtained in so far as the harmonics are not sufficiently attenuated and it does not enable a representation of the thermal state of the installation to be had.
In other known electronic trip devices, a digital infinite impulse response (IIR) filter is used in order to model the thermal state of the installation. If the sampling frequency is sufficiently high, the results obtained are satisfactory. As an example, a filter of this type proves satisfactory for a sampling frequency of 1600 Hz for conventional power systems at 50 Hz, 60 Hz or 400 Hz. If, on the other hand, an inexpensive microprocessor is to be used, which means that the sampling frequency has to be limited, the results obtained are not satisfactory as far as accuracy of measurement is concerned.